Battery Electrode
The present invention concerns the fabrication of carbon electrodes into which alkali metal ions have been electrochemically intercalated. These inventive improved electrodes are particularly suitable for use as negatives for secondary batteries.
Prior art methods for this process require the use of a liquid electrolyte, such as propylene carbonate, and a second electrode that acts as a source of alkali metal ions, either the pure metal itself Dr an intercalated metal oxide. When intercalated metal oxide is used, the configuration is known as a "rocking chair" battery, in which ions rock back and forth between electrodes during charge and discharge of the cell. These batteries have gained much attention in the research community recently, in part due to claims of enhanced safety over conventional cells. Presumably, this derives from the fact that no metallic lithium or sodium is ever in direct contact with the liquid electrolyte in this configuration. However, if rocking chair cells are overdischarged or if the capacities of the electrodes are not carefully matched, metal plating can occur, and the benefits of the configuration are negated.
For the past several years, much attention has been focused on the development of lithium ion type rocking chair batteries. In these cells, a carbon electrode is used as the negative and a lithium intercalating material such as MnO.sub.2 is used as the positive electrode. Such cells are normally assembled in the discharged state (i.e. the positive electrode is already doped with ions) with an appropriate liquid electrolyte and battery separator. Upon charging the cell, the lithium ions move out of the positive electrode and intercalate into the carbon electrode. Upon cell discharge, the reverse process occurs, the lithium moves out from the carbon and into the positive electrode.
Experimental rocking chair cells have indeed shown excellent reversibility, but metal plating can and does occur if cells are overcharged or overdischarged. Thus, the claim of improved safety is, at best, a dubious one, and does not hold under conditions of cell abuse.
Another issue of concern to users is that of the energy density penalty entailed in using the rocking chair configuration. One of the most attractive features of many lithium batteries is their high gravimetric energy densities, a function of the extremely low equivalent weight and density of lithium metal itself.
In spite of these difficulties, however, the rocking chair cells remain attractive alternatives to those using alkali metals, particularly if the dangers associated with plating metal in an organic liquid medium can be avoided. By using a solid polymer electrolyte as the separator and as the binder in both the carbon and the metal oxide electrodes, this difficulty is somewhat ameliorated, and the claim of improved safety for rocking chair cells holds even under conditions of extreme abuse. Additionally, the fact that the electrodes and separator are fabricated into thin, easily handled films simplifies device assembly as well. It is possible to assemble cells into series stacks readily; an option that is not available when liquid electrolytes are used.